Catalyst for purifying exhaust gas

ABSTRACT

Catalyst for purifying exhaust gas comprises manganese oxide as a main catalytic component, calcium aluminate as a binder, and heat-resistant aggregate, wherein the manganese oxide is bonded and solidified onto the aggregate by calcium aluminate(mAl2O3, nCaO), and has an excellent ability to purify the exhaust gas from petroleum combustors or automobiles and a long catalytic life owing to its high heat resistance.

I United States Patent 1 1 1111 3,905,917

Nishino et al. 5] Sept. 16, 1975 1 i CATALYST FOR PURIFYING EXHAUST3.221.002 11/1965 Otzechowski et a] 252/455 R GAS v 3.723.351 3/1973Flank et a]. 252/455 R 3.723.352 5/1973 Alexander et al.. 252/455 RInventors: Msushi i i o; Kazunori i a. 3.779.946 12/1973 Dom et al;2521455 R both of Neyagawa; Hiroshi Kumano, Daito; Yoshinori Noguchl,

Neyagawa a of Japan EXaml ner-Paul F. Shaver i Attorney. Agent, orFirm-Stevens, Davis, Miller & [73] Ass1gnee: Matsushita ElectricIndustrial Co., Mosher Ltd., Kadoma. Japan 1221 Filedi July 20, 19731211 Appl. 190.; 381,232 [57] ABSTRACT Catalyst for purifying exhaustgas comprises manga M" Application nese oxide as a main catalyticcomponent. calciurr June 15, 1973 Japan 43. 3043 aluminatc as a binder,.and heat-resistant aggregate June 15. 1973 Japan 48-68049 wherein themanganese oxide is bonded and solidified onto the aggregate by calciumaluminate(mAl,O;, [52] U.S. Cl 252/455 R; 252/463; 423/213 3 nd has ancellent ability to purify the ex- [51] Int. Cl. B0lJ-29/06; B01] 23/08haust gas from petroleum combustors or automobile: [58] Field at Search252/455 R, 463 and a long catalytic lifie owing to its high heatresistance. [56] References Cited UNITED STATES PATENTS 8 Claims, 4Drawing Figures 3.161.683 l2/l964 Ciucchetti 252/455 R X hair/1w 52,

AVIYAY YIYLVA A A awgyuyaym a a 0 F20 40 so a0 CALCIUM ALUMINATE (at BYWEIGHT) 1 CATALYST roa PURIFYING axnaus'r GAS have been prepared byimpregnating carriers consisting mainly of alumina balls or glass fiberswith a watersoluble metal salt as a catalytic component. and depositingthe metal onto the carrier by thermal decomposition of the metal salt,and metals of group VIII of the Periodic Table such as platinum,palladium. iron, cobalt, nickel etc. as well as their oxides have beenused as the catalytic components..However, these conventional catalystshave the following disadvantages.

The alumina ball or glass fiber carrier is expensive and not economical.lmpregration of the carrier with the catalytic component is verydifi'icult, and the catalytic component is hard to distribute over thecarrier uniformly. The catalyst has a low abrasion resistance due tovibration, and particularly a low spalling resistance at a hightemperature. Consequently, the catalytic life is short.

Metallic oxide catalysts prepared through bondage with a binder such asalumina cement, etc. can be used at a low temperature such as 300 to400C, but their spalling resistance is decreased at a high temperaturesuch as 500C or higher. For example, such metallic oxide catalysts areused as a catalytic component for purifying exhaust gas from automobile,where the cata lyst temperature reaches 800C to 1,000C, a spallingphenomenon appears on the catalyst, and consequently the catalytic lifeis shortened and the abrasion resistance is lowered. i

An object of the present invention is to provide a cheap catalyst ofuniform quality for purifying exhaust gas, which can withstand such ahigh temperature as 400C to 600C and can be utilized for purifyingexhaust gas, for example, from petroleum combustors.

Another objectof the present invention is to provide a catalyst forpurifying exhaust gas, which can withstand such a high temperature as800C to l,0OOC and can be utilized for purifying exhaust gas, forexample, from automobiles.

The catalyst of the present invention consists of manganese oxide as amain catalytic component, calcium aluminate as a heat-resistant binder,and a heatresistant aggregate.

As the manganese oxide as the main catalytic component, manganesedioxide(MnO manganite (M11203), trimanganese tetraoxide (Mn Orl, etc.are used. As MnOz. Y'MnO obtained by electrolysis of an aqueous solutionof manganese salt is used. but natural MnO- ores and chemicallysynthesized MnO, may be also used. However, in view of the quality.uniformity and catalytic activity of the main catalytic component.particularly y-MnO, is preferable. When manganese oxides. MnO, (whereinX 1.3 2.0) are used as a raw material for the main catalytic component,the advantage of using particularly MnO, among other manganese oxidesare given below:

I. It is possible to increase a ratio of manganese oxide in thecatalyst. because, when such lower oxidesias Mn Q', and Mn O, are usedas the raw materialfor the main catalytic component. a catalyst of highbonding strength cannot be obtained unless a ratio of calcium aluminate(mAhQ, nCaO) as a heat-resistant binder is increased. MnO, is anamphoteric oxide, but Mn,0 and Mn O, are basic oxides. Mn,0 and Mn,0,can relatively increase an alkalinity and much accelerate a curing time,but give the catalyst a very high brittleness. contrary to the catalystobtained from MnO,.

2. MnO, is cheaper as, the raw material than Mn,0,, and Mn,-,O,. becauseMnO, undergoes thermal modification as follows:

That is to say, the electrowinning 'y-MnO, or chemically synthesizedB-MnO, or a-MnO, is modified into fl-MnO, at about 250C at first. Then,the fi-MnO, is modified into a-Mn,0, at a temperature of 450 to 650C,and further modified into Mn O, at 950 to l,050C. 0n the other hand,Mn,0, or Mn O, is the basic oxide, and chemically more unstable than MnOFurthermore,, the catalyst based on MnO, undergoes modification at ahigh temperature as mentioned above, and therefore it is not necessaryto use Mn-,.O or Mn O, as the raw materials for the main catalyticcomponent.

3. When a catalyst is prepared from MnO, as the raw material for themain catalytic component. and MnO,

is modified to.Mn O or Mn O, when used, the catalytic activity of thecatalyst is more stable and the catalytic life is longer.

As the calcium aluminate for the heat-resistant binder, the commerciallyavailable, ordinary alumina cement and alumina cement of high aluminacontent can be used. The calcium aluminate has a goodheat resistance, ascomared with the ordinary cement such as portland cement, etc., andfurther its low calcium oxide (CaO) content is more suitable for themanganese oxide. because, if thev binder has a higher calcium oxidecontent, calcium oxide of the binder is liable to react with manganeseoxides as the main catalytic component at a high temperature such as700C or higher to partially form CaMn,O,, etc., and a heat deteriorationof the catalyst is brought about thereby and a high temperatureactivityof the catalyst is reduced. Preferable alumina (A1 0 content ofcalcium aluminate is 50 to percent by weight and calcium oxide (CaO)content thereof 40 to l5 percent by weight.

v 'l'lteheat-resistant aggregates give an important influence upon thespalling. resistance, and particularly the aggregates having a largeheat shrinkage at a high temperature should not be used.

The aggregates used in the present invention include silicate mineralsconsisting mainly of SiO, as a silica aggregate, such as siliceoussands; mullite consisting mainly .ofmAhO}, nSiO, as an alumina-silicaaggregate, such as chamotte (3Al O -2SiO,). sillimanite (M 0, SiO,). andagalmatolite, corundum consisting mainly of M 0, as an alumina aggregatesuch as a-Al- 2 -1 B' s a and 7'': a-

These aggregates can be used alter having been crushed to some extent,or commercially available conical siliceous sand or commerciallyavailable aggregates of alumina or chamotte can be also used.Furthermore. it usually advantageous to use the commercially availablesiliceous sand or chamotte. Further, aggregates of magnesia, chrome.dolomite, magnesiachrome or chrome-magnesia system can he satisfactorilyused. but these aggregates are usually used at a very high temperature.for example. l.300C or higher. and therefore not economical forobtaining a cheap catalyst. when a catalyst is used for purifyingexhaust gas from automobiles. the only requirement for the catalyst is.in general. a good spalling resistance at a maximum temperature ofl.000C. and therefore said silica aggrcgates can meet such requirementsatisfactorily. The ordinary sands. beach sand. etc. are cheap. but thespalling phenomena appear in the aggregates themselves at a hightemperature. and as a result the catalyst breakage takes place.Therefore. the ordinary sand. beach sand. etc. cannot be used at a hightemperature such as l.()C.

When a catalyst is used for purifying the exhaust gas from petroleumcombustors. etc.. the only requirement for the catalyst is a goodspalling resistance at a temperatu re of about 600C. and therefore. theordinary sand. beach sand. etc. which are cheapest among the availablesilica aggregates. can be conveniently utilized.

The distribution of particle sizes of the heat-resistant aggregatesgives also an important influence upon the spalling resistance of thecatalyst. and therefore an optimum particle size must be selected inview of the' desired purpose. use and conditions.

The catalyst of the present invention can be advantageously prepared inthe following manner.

A mixture of manganese dioxide. calcium aluminate and heat-resistantaggregates is admixed with a sufficient amount of water to mold themixture. and then molded into the desirable shape. Only the surfaces ofthe molded articles are dried so that the articles may not stick to oneanother. and then the molded articles are subjected to the primarycuring byleaving the articles standing until the articles have amechanical strength to some extent. Since said amount of water is notsufiicient for complete curing. the articles are then subjected tocomplete curing in water. warm water or steam. i

To give theresulting catalyst a sufficient spalling resistance. it isnecessary to make appropriate the mixing ratio of manganese oxide.calcium aluminate and heatresistant aggregates. curing conditions. etc.Above all. the mixing ratio is especially important. and a ratio of theheat-resistant aggregate must be increased and a ratio of calciumaluminate must be decreased for a high temperature use. i

Now. the present inventionwill be explained in detail by way of examplesand drawings." I

The accompanying drawings show relations between compositions of thecatalyst and spalling resistance.

FIGS. I and 2 show thespalllng resistances at 600C and 700C.respectively. 'of catalysts.=--where beach sands are used as theaggregate.

FIGS. 3 and 4 show, the spalling resistancesat 500C and 900C.respectively. of catalysts. where siliceous sands are used as theaggregate: I i I Table I shows relation between the composition of thecatalyst and the spalling resistance at speeifictemperatures. and theserelations are plotted in ternary catalyst composition diagrams ofmanganese dioxide. calcium aluminate. and heat resistant aggregate asshown catalysts are heated at a temperature of 700Cfor 4 hours.

heating at 600C for 4 hours.

The catalysts of Table l were prepared in the following manner. 7 MnO,.calcium alumiate and beach sand as the heat-resistant aggregate weremixed in the mixing ratio as shown in Table l. admixed with a sufficientamount of water to mold the mixture. molded into cylindrical shapes. 5mmin diameter and I0 mm long. under a pressure of l ton/cm. subjected tothe primary curing for l to 2 hours. then to the secondary caring for 72hours in steam. and dried.

Table l Catalyst composition Spalling resistance uz.ll32 (it by weight)Heating. Heating. Heating. Calcium Mao, uggre- 500C. souc. roux.aluminate gates 4 hours 4 hours 4 hours It) 90 0 '0 O 0 I0 80 I0 0 O 0iii 2O i5 85 (l 0 O i5 5 O O 0 I5 75 l0 0 O 0 I5 70 I5 O 0 0 I5 20 as OO 0 50 ll) 40 '0 o 50 (l 50 '0 O These results reveal the difference inthe spalling resistance at vario'us temperatures. That is. in the caseof heating at 600C for 4 hours. the catalysts comprising the beach sandas the. aggregate can be sufficiently used. but' no specific tendency ofthe mixing ratio is seen at 700C. and almost all of the catalysts are.cracked. .That is. the catalysts comprising beach sands as theaggregatecan sufficiently play their roles within Compositlont'l byweight) Aggregates 9 i Calcium MnO, (beach sand] .aluminate D '5 Is at)F IS 0 i G 40 60 0 H 20 60 20 l 20 50 30 j 5 50 45 E 65 I5 20 0' 5t] 50(l H to so 2n J 20 so 4o The relations between activity of the catalystsand the content of manganese dioxide of the catalyst are given in Table2. The percent purification of carbon monoxide listed in Table 2 wascalculated from concentration of residual carbon monoxide. when thecatalysts as shown in Table l were pulverized to particle sizes of 5 lmesh. 70cc of the pulverized catalysts was packed in a gas passageconsisting of a quartz glass tube. 30 mm in inner diameter. and a gasconsisting of 1.500 ppm carbon monoxide. l5.() percent oxygen and 84.85percent nitrogen was passed through said catalyst layer at allow rate 6r7llrnin.. while keeping the temperature of the catalyst layer constant.I

Table 2 Catalyst composition Percent purification ('Z by weight) of (O(I Calcium MnO, Aggre- 300 C (all? C aluminate Y gates Ill Bil (i 93.098.0

or higher or higher 2i) 70 It) 98.0 98.0

or higher or higher Ill U 30 )(Li 9K0 or higher 2U 2U 60 74.} No.2 2" llso lb!) 34.3

The forgoing results reveal that thecatalysts having a lower manganesedioxide content that percent by weight have a very poor percentpurification of carbon tributes to the percentpurificatiomand thepercent purification is not changed almost at all even by'cha'nging thecontents of calcium aluminate and aggregate. while keeping the manganesedioxide content constant.

It is seen from the foregoing that thecatalysts that can satisfy thespalling resistance and the'cat alytic activity at an intermediatetemperature upto about 600C are within a compositionrangesurroundedi'by,

the lines plotted betweenthe E "H. l a'rid .l of

FIG. I. 1 U

in the foregoing examples, the beach sands'were us'ed as the aggregate.but"the similar results'could be obtained with the ordinary sands. r H

As the binder. calcium aluminatehavin'g about-79.8 percent by weight ofalumina and about 18.7 percent by weight of calcium oxide. the balancebeing [Fe-Q0 and TiO was usedin the foregoing examples. but the similarresult could be obtained with calciurnalurninate having a calciumoxide'co'ntent of 4Q percent by'we ight orless. When calcium aluminatehaving a calcium oxide content of more than 40 percent by weight is usedasthe binder. calcium oxlde oi' the binder isllable to react withmanganese dioxide to form CaMn QO etcJ. catalyst is locally elevatedjoahigh ternpe thermal deterioration of the cataly sjtia brought-about.resulting in failure oflong' use. calcium aluminate having a calciumoxide content of rature and" less than 15 percent by weight is used asthe binder. ti calcium aluminate fails to play a role of a binder for tlmanganese dioxide and the aggregate. and the mecha: ical strength of thecatalyst is much lowered. resultir in failure of practical use. in viewof the bondir strength and heat resistance. the preferable compos tionof calcium aluminum is 50 to 80 percent by weigl of alumina and 40 to l5percent by weight of calciu oxide.

Now, explanation will be made of the composition 1 the catalyst of thepresent invention. which can will stand a high temperature use. Thecatalysts used in ti following examples were prepared by mixing y-MnOcalcium aluminate and siliceous sands (Conical Sil eeous Sand No. 7available from Sanseki Taika Reng K. K.. Japan) as a heat-resistantaggregate in the mixir ratios as shown in Table 3. and carrying out theadd tion of water. molding. primary and secondary curin; and drying inthe same manner as in examples of Tab 1.

Table 3 show relations between the composition 1 the catalyst and thespalling resistances at specific ten peratures. and these relations areplotted in ternary ca alyst composition diagrams of manganese dioxide.ca cium aluminate and heat-resistant aggregate as shou in FIGS. 3 and 4.in H0. 3, the catalysts are heated 1 a temperature of 500C for 4 hours.and in FIG. 4. ti catalysts-are heated at a temperature of 900C forhours. The data at heating at 700C for 4 hours are m shown in a ternarycatalyst composition diagram. b1

the results were quite similar to those of the diagram heating at 900Cfor 4 hours.

Table 3 C atulyst composition Spalling resistance ('1 by weight) calciumMnO, Aggre- Heating. Heating. Heating illuminate gates 500C. 700%, 900C.4 hours 4 hours 4 hour:

to so u 0 O 0 It) so to O O 0 lo 2o 0 O 0 l5 K5 ii 0 Q i5 5 O O 0 I5 75It) 0 O 0 IS 70 l5 0 O 0 i5 20 b5 0 O O 20 so o 0 O I it) 0 I I I 90 olo 0 O between in the case or the heating at 5ooc for 4 how and'th ecat'aly'sts comprising the beach sand as the a gregat'es'canibesuflicientlyused' at that temperatui butthe influence of the aggregatesin large at the he:

iri gat IOW C. The catalysts comprising beach sand the aggregates haveno specific'tendency with respt to mixing ratitaan d undergo almost at aOn 'theo ther handithe c'atalysts comprising the si ceous sands astheaggregate can play their role even Composition ('4 by weight) PointsAggregates Calcium (conical MnO siliceous aluminate sands D 5 l 5 80 F85 l 5 G U 50 (i l 50 J 20 40 40 K it) 40 i. it) 20 M 5 20 E as l 5 20 H30 5O 20 The relations between the activity of the catalyst and themanganese oxide content of the catalyst are shown in Table 4. Thepercent purification of carbon monoxide in Table 4 was calculated in thesame manner as in Table 2.

Table 4 Catalyst composition Percent purification (A by weight) of CO(54) Calcium MnO, Aggre- 300C 800C aluminate gates or higher or higher20 70 ll) 93.0 93.0

or higher or higher 20 .50 30 92.7 98.0

or higher 20 20 so 73.l NJ 20 0 "L3 46.)

The foregoing results reveal that at a low temperature such as 300C. thecatalyst having a manganese dioxide content of less than 20 percent byweight has a very poor percentpurification. for example. less than 20percent, whereas the catalyst having a manganese dioxide content of 20percent byweight has a percent purification of more than 70 percent.Further. at a high temperature of 800C. the catalyst having evenamanganese dioxide content of less than percent. by weight has animproved percent purificationpbutthe percent purification of lessthan;50 percent is notsatisfactory. The catalyst having amanganese'dioxidecontent of more than 20 percent byweight has a percentpurification of more than percent. and thiis has a good catalyticperformance. The manganese dioxide content greatly contributes to thepercent purifiction. and the percent purification is not changed almostat all even by changing the contentsof calcium aluminate and aggregates.while keeping the manganese dioxide content constant.

it is seen from the foregoing that the catalysts that satisfy thespailing resistance and catalytic activity at a high temperature arewithin a composition range surrounded by lines plotted between thepoints D. E. H. I. .I'. K. Land M ofFlO. 4.

in the foregoing examples. conical siliceous sands of silica aggregateswere used as the heat-resistant aggregates. but similar results could beobtained with the aforementioned alumina and silica-alumina aggregates.

Calcium aluminate containing about 79.0 percent by weight of alumina andabout 18.7 percent by weight of calcium oxide. the balance being Fe O,and TiO,. was used as the binder. but the similar results could beobtained with calcium aluminate having not more than about 40 percent byweight of calcium oxide.

in the foregoing example. MnO, was used as the manganese oxide as a maincatalytic component. but as to the spailing resistance at a hightemperature. almost similar results of cracking strength as in the caseof MnO, were obtainedwith the catalysts prepared from MnO, (x 1.5 1.33).for example. Mn O Mn;.0., etc. This means that. when such manganescoxides as Mn q, and Mn are used as a raw material for the main catalyticcomponent. the similar results as in the case of MnO, used as the rawmaterial can be obtained so long as the manganese oxide content is witinsaid stoichiometrical composition range in tenns of MnO though themanganese content differs at the normal temperature.

in addition to the main catalytic component of the manganese oxide. notmore than 20 percent by weight of at least one kind of promoters can beadded to the catalyst to improve the low temperature activity As thepromoter. NiO mo... Cu-,O.. CuO. V 0 C0 0 C0 0... PbO. TiO AggO.,RgO(rare earth oxides) etc. are used. depending upon the purpose and use ofthe catalyst. v

in the foregoing Examples. no mention is made of hydrocarbon. but thepercent purification of hydrocarbon generally shows a tendency almotidentical with that of carbon monoxide.

As described above. the present catalyst for purifying the exhaust gasis advantageously prepared by adding to a mixture of manganese oxide.calcium alu minate and aggregates a sufficient amount of water to moldthe mixture. mixing the mixture by wet process. molding the mixture.subjecting the moldings to the primary curing. then addinga sufficientamount of water to the moldings and subjecting the moldings to completecuring and solidificatiomAccording to this procedure. the

catalysts having any desired shape can be readily ob tained. Thereforethe present catalysts can be prepared in any desired shape such asgranular shape. honeycomb structure. etc. in mass production scaleaccording tothisprocedurc. depending upon the service purpose and of thecatalyst.

Further. the present catalysts canbe prepared by the molding'and curing.and thus their preparation are easy and the catalysts of desiredconstant quality or composition can be obtained. Thecataliysts thusprepared always have a constant gas purification ability. even if thesurfaces of the catalysts are abraded. because the parts to. besuccessively exposed by theabrasion are constant in quality ofcomposition. Further. the present catalysts also have u good spailingresistance. as described above. r y r The present catalysts can used forpurification of carbonmonoxide and hydroearbonscontained in the exhaustgas from engines of automobiles. petroleum stoves. or various othercombustors by properly selecting the molding process or particle sizes.

What is claimed is:

1. A catalyst for purification oi exhaust gas. which consistsessentially of manganese oxide as a main catalytic component.heat-resistant aggregates and calcium aluminate as a binder. proportionsof the manganese oxide in terms of manganese dioxide, thc a'g gregatesand the calcium aluminate beingwithin a composition 'zrange surroundedby lines plotted between points D, E," H, l and J of ternary catalystcomposition diagramof FIG. I.

2. A catalyst according to claim I. where the calcium alumlnate contains50 to 80 percent by weight of alumina and 40 to is percent by weight ofcalcium oxide.

3. A catalyst according to claim 1. wherein the aggre gates are selectedfrom the group consisting of silica ag-, gregates. alumina-silicaaggregates. alumina aggregates. and mixtures thereof. i f

4. A catalyst according to claim I, wherein the portions of themanganeseoxide in terms of manganese dioxide. the aggregates and the,calcium aluminate are within a composition range surrounded by linesplotted between points D. E, H, l. .I'. K. L. and M of ternary catalystcomposition diagram of FIG. 4.

5. A catalyst according to claim 4, wherein the cal cium aluminatecontains 50 to 80 percent by weight alumina and 40 to percent by weightof calcium ox idc.

6. A catalyst according to claim 4, wherein the aggre gates are selectedfrom the group consisting of siliceou: sands. muilite. sillimanite.agalmatolite. oorundum. anc mixtures thereof.

7.A process for producing a catalyst for purifying exhaust gasconsisting essentially of manganese oxide as a main catalytic component.heat-resistant aggregates.

ganese oxide is manganese dioxide.

i i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patan:No.,917 Dated September 16, 1976 hwenum s Atsushl Nlshlno et al. P 1 f 4 Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Insert FIG. 1, FIG. 2, FIG. 3 and FIG. 4, as part of Letters Patent3,905,917

-zo a Q, Aernee p m gm a, o Y? Y Y 'AYAYA.

("A A "56 v 56 50? 80 IO CALCIUM ALUMINATE BY WEIGHT) Patent 7 DatedSeptember 16, 1976 Invenwfls) Atsushl Nishino et al. Page 2 of 4 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

A AWYAYAR O 20 4O 6O 80 I00 CALCIUM ALUMINATE BY WEIGHT) UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,905,917 DatedSegtember 16, 1976 lnventofls) Atsushi Nishino et al. Page 3 of 4 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby cerrected as shown below:

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N 3,905,917Dated September 16, 1976 Inventods) Atsushi Nishino et al. Page 4 of Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

46' LCIUM ALUMINATE BY WEIGHT) 0 Signed and Scaled this Third Day of-Maj 1977 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Ar esting Ulfi'ce Commissioner oflatemsand Trademarks

1. A CATALYST FOR PURIFICATION OF EXHAUST GAS, WHICH CONSISTSESSENTIALLY OF MANGANESE OXIDE AS A MAIN CATALYTIC COMPONENT,HEAT-RESISTANT AGGREGATES AND CALCIUM ALUMINATE AS A BINDER, PROPORTIONSOF THE MANGANESE OXIDE IN TERMS OF MANGANESE DIOXIDE, THE AGGREGATES ANDTHE CALCIUM ALUMINATE BEING WITHIN A COMPOSITION RANGE SURROUNDED BYLINES PLOTTED BETWEEN POINTS D, E, H, I AND J OF TERMARY CATALYSTCOMPOSITION DIAGRAM OF FIG.
 1. 2. A catalyst according to claim 1, wherethe calcium aluminate contains 50 to 80 percent by weight of alumina and40 to 15 percent by weight of calcium oxide.
 3. A catalyst according toclaim 1, wherein the aggregates are selected from the group consistingof silica aggregates, alumina-silica aggregates, alumina aggregates, andmixtures thereof.
 4. A catalyst according to claim 1, wherein theproportions of the manganese oxide in terms of manganese dioxide, theaggregates and the calcium aluminate are within a composition rangesurrounded by lines plotted between points D, E, H'', I, J'', K, L, andM of ternary catalyst composition diagram of FIG.
 4. 5. A catalystaccording to claim 4, wherein the calcium aluminate contains 50 to 80percent by weight of alumina and 40 to 15 percent by weight of calciumoxide.
 6. A catalyst according to claim 4, wherein the aggregates areselected from the group consisting of siliceous sands, mullite,sillimanite, agalmatolite, corundum, and mixtures thereof.
 7. A processfor producing a catalyst for purifying exhaust gas consistingessentially of manganese oxide as a main catalytic component,heat-resistant aggregates, and calcium aluminate as a binder, whichcomprises adding to a mixture of manganese oxide, aggregates and calciumaluminate, a sufficient amount of water to mold the mixture, mixing thewet mixture and molding the mixed mixture to form moldings, allowingsaid molded articles to stand to undergo primary curing until thearticles have a mechanical strength, and completely curing the moldingsby adding sufficient water to solidify said moldings.
 8. A processaccording to claim 7, wherein the manganese oxide is manganese dioxide.